Preheat oven to 400 degrees. Place sheets of puff pastry on a baking tray(s). Prick surface with a fork

a rack to cool

Prepare onions, heat a large skillet over medium heat. Add olive oil, onions, thyme and a pinch of salt. Cook, string occasionally, until onions turn golden and sweet, about 15 to 20 minutes.

In a small bowl, mix mustard with cream. When pastry is slightly cool, return to baking tray and spread with mustard mixture just to the 1/2 inch border. Scatter half of the cheese over sauce and top with onions and sun-dried tomatoes, top with remaining cheese.

Return tray(s) to oven and bake until cheese melts and browns in spots about 15 minutes.